LAS VEGAS — The heavyweight unification series, with only slightly fewer contenders than the NCAA basketball tournament, will begin tonight with Pinklon Thomas, the World Boxing Council champion, defending against top-ranked Trevor Berbick.

This particular fight will not actually unify anything, but intentions are good. So it fits, in the logic of boxing.

In all, there will be at least seven fights in the next year designed to reduce the list of contenders and champions, at least two of them pretenders, to one. Someday, believe it or not, heavyweight boxing will be without all those competing initials--WBC, WBA, IBF--that make the game look as much like an eye chart as anything else.

Tonight's fight, at the Riviera hotel and casino, will be a start. And, some folks believe, it will be the finale as well, since Thomas, at the moment, looms as the dominant champion. Those folks think that the 28-year-old former heroin addict-turned-boxer-turned-singer will be the man still standing when this HBO-sponsored tournament is over.

Certainly Thomas, whose record is besmirched only by a draw with Gerrie Coetzee, is the favorite tonight. Probably that's because Thomas, alone among the heavyweight champions, has made a successful title defense.

That hardly qualifies him as one of the sport's legends but, then, the level of heavyweight boxing has declined to the extent that even one successful defense can be viewed as a long and wonderful career.

The other reason that Thomas is such a favorite is that Berbick, the 31-year-old challenger, is unpredictable. He extended former champion Larry Holmes a full 15 rounds, retired Muhammad Ali, destroyed David Bey. Then he lost to Renaldo Snipes and, in possibly the worst fight ever made, lost to S.T. Gordon. He said he was drugged, but that's another story.

Thomas, who of late has been billing himself as the Fighting Preacher, is nevertheless a tough fighter, the kind who wears people down with an unremitting attack to the body.

"He'll be going through some changes," Berbick predicted for Thomas Friday at his pool-side weigh-in.

Of course, Thomas has already been through some changes. It's a familiar story in boxing circles how Thomas, with the help of his wife, Kathy, overcame teen-aged heroin addiction and survived the streets of Detroit and Seattle to become a boxer. Even without that angle, it is almost a miracle for somebody who first laced on gloves at the late age of 19 to become a champion.

And now he's going through even more. Certainly he intends to win this tournament and restore order to the once prestigious division. But he has longer range goals as well.

"Here, listen to this and let me know what you think," Thomas says, slipping you a cassette tape. You listen, half afraid to be embarrassed for him, and you are pleased. It may just be the dancing bear factor--the wonder is not that the bear dances well, but that it dances at all--but boxing writers, who may not be the best judges, think that Thomas sings surprisingly well.

Then there is his managing career. Thomas, who is so independent that he serves as his own manager--he is now without even trainer Angelo Dundee in his corner--manages two young fighters he found in a Philadelphia gym. "I want to be getting into business, too," he says, his ambitions vague but strong.

Possibly in consideration of the fullness of his future, he has his telephone service answer: "Pinklon Thomas, Incorporated."

In the meantime, though, there is this little tournament, co-promoted by HBO and Dynamic Duo, the latter of which is a rare collaboration of rival promoters, Don King and Butch Lewis. The reason King doesn't have the division to himself as usual is that Lewis has the International Boxing Federation champion, Michael Spinks, the man who upset Larry Holmes last year.

That rematch, scheduled for April 19 at another hotel here, has somewhat overshadowed tonight's fight. And the fact that tonight's fight was scheduled so soon after the recent middleweight fights here also has hurt it as an attraction. Evidently the public can endure just so much boxing.

Thomas, who will get $635,000 to Berbick's purse of $50,000, said, for that matter, that there is just so much of Berbick he can endure, like about six rounds. "I don't think it will even go six," he said.

He has promised that he won't keep his vaunted jab hanging in the air and will force the fight. "I'll be on him like a swarm of bees," Thomas said. "Won't be no place for him to hide, unless he can fit in a corner bucket."

Thomas is eager to move on and conclude this tournament and is understandably confident that he can do so, since he already holds a win over the World Boxing Assn. titlist, Tim Witherspoon. Of course the division is unpredictable enough that he can't count on Witherspoon still being the WBA champion by next summer when the alphabet champions collide.

In fact, all you can count on for next summer is that there will be one champion, for the first time since 1978, when Muhammad Ali ruled a glory division.